Distopiary Ruby & Eater

In a world where a person's Post is decided for them from birth, Torza holds the most powerful, the most punishing, and the most heroic of Posts--he is a Slayer. He sets off on a journey to gather strong companions for the sole purpose of defeating the twenty-third incarnation of the all-powerful Demon. However, there's a dark truth lurking behind the stories of the so-called heroic Slayers...

(Source: Yen Press)